The Section on Neuroendocrinology conducts research on the pineal gland with heavy emphasis on the biochemistry and molecular biology of the two enzymes involved in the conversion of serotonin to melatonin. This indolic compound is synthesized at high levels at night in all vertebrates, due to a marked increase in the activity of the first enzyme in the serotonin melatonin pathway, serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT, EC2.3.1.87). The Section has determined the genetic code of the ovine, rat, human, and chicken forms of the enzyme. In all cases mRNA encoding the enzyme is about 1.0 to 1.5 kb. The human gene is about 2.5 kb. mRNA encoding this enzyme is found at high levels consistently in the night pineal gland; lower levels are found in the retina of most species. In addition, trace amounts are found in certain brain regions and in the pituitary gland. The levels of mRNA in the pineal gland exhibit a about 150-fold night/day rhythm in the rat, in which day values are nearly undetectable. The rhythm in the chicken is about 10- to 15-fold and that in the sheep is not more than 1.5 fold. Expression of the AA-NAT gene in the rat is regulated by a cyclic AMP mechanism, in which expression is turned on by phosphorylation of a cyclic AMP response element binding protein. This induces expression of AA-NAT and coinduces the expression of genes encoding two inhibitory transcription factors, at least one of which seems to play a role in rapidly turning off expression of the AA-NAT gene. Analysis of the protein encoded by the AA-NAT gene reveals that it is distantly related to a family of acetyltransferase molecules and that AA-NAT represents a new subfamily.